The present invention concerns a precision current measurement device, typically involving a Rogowski coil.
Rogowski coils are well known electrical devices finding use today for measurement of magnetic fields and electrical currents. They have been researched over the past century and are well known to the scientific literature. Their origin traces to the invention circa 1912 of the Rogowski coil by W. Rogowski and W. Steinhaus. The device is useful for measuring electrical currents and operates on the basis of a magnetic field integration performed across a closed contour being proportional to the current flowing through the contour. The coil provides a voltage output that is proportional to the time derivative of the current (di/dt) rather than a current output like other current transformers.
Rogowski coils are popular because of their dynamic range and linearity. However, though theoretical requirements are known, manufacturers still need ways to provide a high quality coil that is both economical to manufacture and which is satisfactory for precise current measurements. The device (coil) should be insensitive to external influences, insensitive to the measured primary conductor position, and retain high precision (in the order of 0.3% or better) over its lifetime and across a wide temperature range (nominally −40 to 85 degrees Centigrade).
There are several major requirements for implementing successful Rogowski Coil implementations. Return path cancellation, geometric symmetry, and electrostatic shielding are major requirements. A true Rogowski coil implementation must include a return path to eliminate the undesirable magnetic loop area created by the advancement of the coil winding. Many previous “Rogowski coil” implementations either ignore the return path compensation as in U.S. Pat. No. 5,852,.395 or provide a poor approximation of the return path as in U.S. Pat. Nos. 6,437,555 and 5,414,400. In addition, several of these previous attempts at providing return path compensation are more difficult to manufacture, such as, for example, UK Patent Application number 2,259,150 to Frederick.
When the return path is omitted for a “Rogowski coil” device, it is commonly referred to in the art as a linear coupler, not as a Rogowski coil. When the return path is omitted or poorly approximated, the device exhibits excessive measurement sensitivity to the position and angle of the conductor within the device opening and susceptibility to magnetic fields produced by electric currents that are external to the device. Susceptibility to these externally generated magnetic fields will hereafter be referred to as “external magnetic field susceptibility.”
Geometric symmetry of construction is another factor that contributes to positional and angular sensitivity of the placement of the conductor within the device opening and external magnetic field susceptibility. The individual conductor turns that make up the windings must be placed symmetrically, around the perimeter of the coil and concentrically around an imaginary axis in the center of the coil's conductor turns. In addition, the core must be fabricated with a method and material that prevents warpage and that provides for concentric thermal expansion.
With sufficient geometric control and return path cancellation, a Rogowski coil can provide very high accuracy at a specific temperature. Even with the best geometric control, however, the magnitude of the Rogowski coil output still remains sensitive to temperature change due to the coefficients of thermal expansion of the utilized materials. Nevertheless, as a result of sufficient geometric control, it is possible to temperature compensate the device output for a desired level of accuracy across a specific temperature range without incurring undesired positional sensitivity, angular sensitivity, or external magnetic field susceptibility.
Prior art exists that attempts to temperature compensate printed circuit boards; however, the printed circuit board materials have very poor Z-axis expansion and have poor thickness variation control. Common printed circuit boards are not monolithic, but are actually constructed by laminating numerous thin “prepreg” sheets (partially cured, fiber reinforced sheets that are preimpregnated with a resin) together. This construction can cause significant geometric control problems. As a result, temperature compensation is only partially effective without some other process to provide sufficient geometric control. In order to manage the relatively poor tolerances of the printed circuit board materials, a special culling process is required in order to obtain materials that do not have excessive thickness variation in the cross-sectional area.
One example of a Rogowski coil using a printed circuit is disclosed in U.S. Pat. No. 5,414,400 entitled “Rogowski Coil” which discloses a Rogowski coil made on a printed circuit plate provided with a circular cut-out. The coil is implemented by metal deposits on each of the two faces of the plate extending along radii, with electrical connections between the radii on one face and those on the opposite face being achieved via plated-through holes passing through the thickness of the plate.
U.S. Pat. No. 5,442,280 discloses a method for manufacturing a printed circuit board-based Rogowski coil. The disclosed geometry provides very high turn density resulting in very high sensitivity. While high sensitivity is very desirable when measuring low frequency currents (50/60 Hz power system related), the patent fails to provide adequate means for external magnetic field cancellation This problem is reported in U.S. Pat. No. 6,624,624 and is caused by inadequate handling of the coil return path.
The relatively long conductor of the coil winding is subject to electric field exposure in nearly any real world environment. As a result, electrostatic shielding must be included to prevent the winding from acting as a wideband antenna. Otherwise, the Rogowski coil output signal can be corrupted by surrounding sources of electromagnetic interference appearing as noise. It is known to provide shielding around a coil. For example, Kaczkowski U.S. Pat. No. 6,288,625 entitled “Current Transformer For A Metal-Enclosed Gas-Insulated Hi-Voltage Installation” includes electrically conducting walls that act as a “screen” around a pickup coil that is wound around an annular core and surrounded by an electrically insulating compound. See also EP0889490 (Kaczkowski) and having the same title.
Von Skarczinski et ale U.S. Pat. No. 5,982,265 is directed to a current detection coil for a current transformer. This says that it uses a Rogowski coil with an insulating annular body made of a fiber-reinforced thermosetting plastic mounted in a flame or housing. This patent does not disclose a return path for the coil.
A similar problem applies to the design reported in U.S. Pat. No. 6,313,623 (by one of the present inventors) in which two closely spaced coils with counter rotation are used to perform partial return path compensation.
Further attempts to design precision Rogowski coils are disclosed in U.S. Pat. No. 6,624,624. Attempts to provide improved return path cancellation resulted in significantly reduced coil densities, making the design less appropriate for low frequency current measurement applications. In addition, although improved, all reported geometries suffer from Z-axis (board thickness) related sensitivity effects with an error cancellation (return) path normally offset in the direction of the Z-axis (board thickness).
J. D. Ramboz in “Machinable Rogowski Coil, Design and Calibration,” IEEE Transactions on Instrumentation and Measurement, Vol. 45, No. 2, (April 1996) pp 511-15 reviews traditional designs for Rogowski coils and discusses a “machinable” Rogowski coil constructed using machinable ceramic material to make a toroidal coil with a rectangular cross section. A thin, electrically conductive coating is then applied to the coil, totally encapsulating the ceramic core. Next, thin lines of the conductive material are removed by laser machining methods in a pattern that leaves coils as bands of conductive material located radially around the core. Each turn or band was connected to the next turn by a suitable indexing.
U.S. Pat. No. 6,300,857 for “Insulating Toroid Cores and Windings” discloses a configuration to improve the winding of precise conventional transformer coils and includes an insulating jacket around a magnetic core. The insulating jacket includes plural protrusions around the core, the protrusions demarking various segments of the toroid. For example, the toroid may be divided into six evenly spaced sections, each occupying approximately 60°. At the edges of each section, there is a protrusion. The protrusions maintain the placement and spacing of windings within each section.
An object of the present invention is to provide a precision di/dt transducer that addresses various problems of the prior art to provide a precise, practical, and manufacturable electric current measuring device using a Rogowski coil.
A further object is to improve the core structure for a Rogowski or other coil.
Yet another object is to provide an improved shield for Rogowski or other coils.